I. FIELD OF THE INVENTION
This invention relates to the treatment of used linings of aluminum reduction cells, commonly referred to as spent pot linings. More particularly, the invention relates to the treatment of linings of this type to permit safe disposal of the linings and to recover useful values contained in the linings.
II. DESCRIPTION OF THE PRIOR ART
Aluminum is normally produced by dissolving alumina at high temperature (above 900.degree. C.) in molten cryolite (Na.sub.3 AlF.sub.6) in electrolytic cells, or pots, provided with electrically conductive carbon linings, and electrolyzing the molten solution by passing an electric current between carbon anodes dipping into the melt and the carbon linings acting as cathodes. Cells of this type may be used for considerable periods of time, e.g. up to seven years, and during this time the carbon lining material absorbs sodium fluoride and other contaminants. At the end of the operational lifetime of the cells, the linings are removed and broken up and have to be disposed of in some way. However, the spent lining material, which is composed of carbon, refractory material from insulating refractory bricks and cryolite, including fluorine, aluminum, sodium, calcium and silicon values, along with free and complexed cyanides, carbides and nitrides, is hazardous and must be treated with great caution. Not only are inflammable and explosive gases (particularly hydrogen, methane and ammonia) generated if the linings are contacted by moisture, but also the used lining material contains highly toxic cyanides. The safe disposal of spent linings has therefore presented a challenge to the industry and several methods and schemes have been proposed, as briefly indicated in the following.
Several proposals involve thermal treatment of the linings, e.g. in fluidized beds (as disclosed for example by Tabrey et al in PCT publication WO 90/13774) or rotary kilns (e.g. as disclosed by Banker et al in U.S. Pat. No. 5,164,174). Such methods are intended to destroy the cyanide contents and to convert fluoride values to insoluble salts. However, their disadvantage is that they do not convert the carbon, aluminum and fluoride values into forms that can be recycled and the treated material is suitable only for dumping in land-fill sites. This renders the processes uneconomical.
Another approach involves pyrohydrolysis and pyrosulpholysis, e.g. as disclosed by Bell et al in U.S. Pat. No. 4,113,832. These processes require large expensive reactors and their high capital and operating costs make them uneconomical to operate.
Fixation in a glassy matrix has also been attempted as disclosed by Bontron et al in Canadian Patent 2,055,135, but again this is only capable of converting the material into forms which can be dumped safely in landfill sites.
Gardner et al in U.S. Pat. No. 4,927,459 have provided a way of using the material as a flux and fuel in secondary smelting. Although this disposes of the material, the sodium, aluminum and fluoride values are lost.
Leaching in caustic solutions, either with or without a preliminary heat treatment has also been suggested. Methods involving caustic leaching after a heat treatment are exemplified by Snodgrass et al in U.S. Pat. No. 4,444,740; Rickman in U.S. Pat. No. 4,973,464; Goodes et al in U.S. Pat. No. 4,900,535; Jenkins in WO 92/12268; and Hittner et al in U.S. Pat. No. 5,024,822. Leaching in alkaline solutions made from lime without preliminary heat treatment is exemplified by Gnyra in British patent application GB 2,056,425 and by Kaaber et al in WO 92/13801.
Unfortunately none of these proposals is capable of recovering the carbon and valuable constituents of the potlining material while also dealing safely with the cyanide content, at least in an economical and relatively simple manner.
There is therefore still a need for a safe, economical and relatively simple way of recycling spent potlining materials in equipment that does not require undue capital outlay.